Kriyaa Hi Vastoopahutaa Praseedati

Indian and World Geography, Study Materials

Universe and Solar System

The Universe

The Universe is all of space and time and their contents, including planets, stars, galaxies and all other forms of matter and Energy.

Evolution of Universe: There are three main theories put forward to explain the origin and evolution of the Universe are: –

  • The Big Bang Theory: In 1916, Einstein published his theory of general relativity with this he also proposed a theoretical model of the Universe which was not expanding.
  • 1922 Russian cosmologist Alexander Friedman developed what is known as the Friedman equations which were derived from Einstein equations for general relativity: contrary to Einstein was advocating at the time with his cosmological constant. Friedman work showed that the Universe was likely in a state of expansion.
  • 1924, Edwin Hubble’s measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies.
  • 1927, Georges Lemaitre, a Belgian physicist and Roman Catholic Priest independently derived the same result as Friedman Equations and proposed that the inferred recession Galaxies was due to the expansion of the Universe.
  • 1929, Hubble discovered a correlation between distance and decussation velocity, which is known as the Hubble law.
  • 1931, Georges Lemaitre was suggesting that the current expansion of the Universe meant that the farther back in time, one went, the smaller the universe would be. At some point in the past, he argued, the entire mass of the Universe would have been concentrated into a single point from which the very fabric of space and time originated.
  • 1949, Fred Hoyle coined the phrase “Big Bang” for Lemaitre Hypothesis.
  • 1965: The discovery and confirmation of the cosmic microwave background Radiation (M.B.R) practically supported the Big Bang theory.
  • 1981, Physicist Alan bath theorized of a period of rapid cosmic expansion that resolved other theoretical problems.
  • 1990, the discovery of the rise of Dark energy resolves many issues of Big Bang theory.
  • Some other experiments also supported the Big Bang theory.
    • Cosmic Background Explorer (COBE)
    • Hubble Space Telescope
    • Wilkinson Microwave Anisotropy Probe (WMAP)
    • Planck observatory
    • Large Hadron collider experiment (LHC)
  • According to Big Bang theory at starting the whole mass of the Universe concentrated in an extremely dense and hot fireball like structure 13.7 billion years ago, it exploded (Big Bang). The matter was broken into pieces, which were thrown out with high speed in all directions forming stars and galaxies which are still moving away from one another.


A galaxy is a huge collection of gas, dust and billions of stars and their solar systems. A galaxy is held together by gravity.

Nebula: It is a Latin word which means mist or cloud. Nebulae are not only massive clouds of dust, hydrogen and helium gas and plasma: they are also often Steller nurseries – i.e., the place where stars are born. A Nebula is formed when a portion of the interstellar medium undergo gravitational collapse. Mutual gravitational attraction causes matter to clump together, forming region of greater density. Its ultraviolet ionizing radiation causes the surrounding gas to become visible at optical wavelength. Orion sward is nebulae of our galaxy. Nebula divided into four categories:

  1. Diffuse Nebulae
  2. Dark Nebulae
  3. supernova Remnant Nebulae
  4. Planetary Nebulae

Quasars: Quasi-stellar radio sources or Quasars was discovered in 1960 by Allan standage. Quasars are among the brightest and most distant known celestial objects and crucial to understanding the early Universe. Quasars live only in galaxies with supermassive Black holes.

Black Holes: These are infinitely dense points in space with deep gravity sinks so that not even light can escape the powerful tug of its gravity. Therefore, it is also known as the cosmic vacuum cleaner. Anything that ventures too close will be stretched and compressed like putty in a theoretical process aptly known as spaghettification.

Constellation: A constellation is a group of stars that forms definite imaginary outline or pattern on the celestial sphere. 89 constellations known in our galaxy. Hydra is the largest constellation according to the area and smallest constellation according to number of stars (68 stars) The crux is the smallest constellation according to the area. Centaurus is the largest constellation, according to the number of stars. (92 stars) Most popular constellations is Ursa major which lies in the northern sky. In Latin, it is also known as the Great Bear or the larger Bear. In Indian mythology, it is known as Saptrishi.

Pole star: In the northern hemisphere, it is also known as the north star. It is the brightest star that appears nearest to either celestial at a particular time. At present, the pole star is Polaris (α Ursa minoris). 2700 BCE Theban (α Draconis) is our pole star. 1400 CE Vega become our pole star. The present southern pole star is Polaris Australis (α octant is).


Stars are big exploding balls of gas, mainly Hydrogen and helium; which secretes a huge amount of energy. Sun is the nearest Star of Earth. Proxima Centauri is the nearest Star of Sun. Star colour is linked to temperature.

Life cycles of stars: A star’s life cycle is determined by its mass. The larger it mass, the shorter its life cycle.

  • Molecular Star: Stars start out as vast clouds of cold molecular gas. The gas cloud could be floating near a galaxy for millions of years, but then some event causes it to begin collapsing under its own gravity. For example, when galaxies collide, regions of cold gas are given the kick they need to start collapsing. As it collapses, the interstellar clouds break up into smaller and smaller pieces, and each one of these collapses inward on itself. Each of these pieces will become a star.
  • Proto star: As the Steller material pulls tighter and tighter together, it heats up pushing again further gravitational collapse. At this point, the object is known as Proto star.
  • T Tauri star: At Tauri star begins when material stop falling into the protester and it is releasing a tremendous amount of energy. The T Tauri phase lost for about 100 million years.
  • Main sequence: It starts when the core temperature of a star will reach the point that fusion its core can begin (Exothermic reaction).
  • Red Giant: When a star exhausts its fuel of Hydrogen at its core, its internal nuclear reactions stop. Now star begins to contract inward through gravity. This process heats up a shell of Hydrogen around the core which then ignites in fusion and causes the star to brighten up again. This causes the outer layers of the star to expand outwards, increasing the size of the star many times. The temperature and pressure at the core of the star will eventually reach the point that helium can be fused into carbon and known as red Giant/ dwarf.
  • White dwarf: The Star will eject its outer layers into space and then contract down, eventually becoming a white dwarf.
  • Supernova: It is a giant dying Star of high mass that has come to the end of its life by a spectacular explosion.
  • Neutron star: It is the incredibly compact core that remains after a supernova explosion.

Solar system

The solar system contains the Sun, eight planets with their satellites, dwarf planets,
asteroids, comets and other near-earth objects.

The Sun

  • Our Sun is a normal main-sequence G-2 yellow dwarf star, a hot ball of glowing gases, one of more than 100 billion stars in our galaxy. Its gravity holds the solar system together, keeping everything from the biggest planets to the smallest particles of debris in its orbit.
    • Diameter – 13,90,000 km;
    • Mass – 1.989e30 kg;
    • Temperature – 5800 k (surface) 15,600,000 k (core)
  • The Sun is personified in many mythologies; the Greeks called it Helios, and the Romans called it sol.
  • Sun contains more than 99.8% of the total mass in the solar system.
  • The Sun is primarily made up of hydrogen (H) (70%) helium (He) (28%) together with a small amount of carbon, oxygen, iron, neon and other elements.
  • The outer layers of the Sun exhibit differential rotation → At the equator the surface rotates once every 25.4 days – and near the pole, it is as much as 36 days. This odd behaviour is due to its gaseous structure.
  • The core of the Sun covers approximately the inner 25% of its radius. At the centre of the core the sun density is more than 150 times that of the water.
  • The Sun’s power is produced by the nuclear fusion reaction. Each second more than 70 million tons hydrogen undergo this process and produce energy in the form of Gamma Radiations.
  • The surface of the Sun called photosphere (5800 k).
  • Sunspots are cool regions (3800 k)
  • A small region known as the chromosphere lies above the photosphere
  • The highly rarefied region above the chromosphere called the corona, visible during a total solar eclipse.
  • The outermost part of the Sun is known as the coronal sphere.
  • The Sun’s magnetosphere/Heliosphere (magnetic field area) extends well beyond Pluto.
  • Sun also emits low density streams of charged particles like electron, protons etc known as solar wind which propagates throughout the solar system. During the solar minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate. During the solar maximum the solar wind moves at an intermediate speed.
  • The age of the Sun is 4.6 billion years.


According to the International Astronomical Union Summit 2006; a planet is a celestial body that:

  1. Is in orbit around any star
  2. Having area at least 3000 km2
  3. Got their energy and light from that star.
  4. Has sufficient mass for itself gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape (Nearly round)
  5. Has cleared the neighborhood around its orbit

In our solar system, the total number of planets is 8; which is divided into two categories:

  1. Terrestrial or Inner Planet – They found inside asteroids belts. They are 4 in number: Mercury, Venus, Earth, Mars.
  2. Gas Giant or Outer Planet – They are 4 in number Jupiter, Saturn, Uranus, Neptune.

Planets which orbit other stars (not the Sun) are called exoplanets.

1. Mercury

  • The closest planet to the Sun and a very hot planet.
  • Smallest Planet in the solar system with a diameter of 4900 Km.
  • Fastest Planet with a speed of 172500 Km per hour to complete revolution around the
    Sun in 88 days.
  • The Planet with no water and gases like Nitrogen, Hydrogen, Oxygen and Carbon Dioxide.

2. Venus

  • Hottest Planet in the solar system with the surface temperature of 478 degree Celsius.
    Also known as “Earth’s Twin”. It is because of similarity in size and mass between Venus and Earth.
  • One of the two planets in the solar system which rotate around the axis in a clockwise direction.
  • The brightest star in the Solar system. It can be seen in the morning and evening with open eyes. So known as “Evening Star” and “Morning Star”.

3. Earth

  • The only Planet to give support to life with a pleasant atmosphere.
  • Also known as “Blue Planet” because of the presence of water on it.
  • It has one natural satellite named “Moon”.

4. Mars

  • Known as “Red Planet” because of Iron-rich red soil.
  • Second smallest Planet in the solar system after Mercury.
  • Has two natural moons “Phobos” and “Deimos”.
  • Has a thin atmosphere and surface with valleys, craters, deserts and ice caps etc.
  • “Olympus Mons” – Largest volcano and the tallest mountain in the solar system lies on Mars.

5. Jupiter

  • Largest Planet of the solar system with the shortest rotation
  • Has atmosphere filled with Hydrogen, Helium and other gases
  • The third brightest object in the night sky after the Moon and Venus.
  • Great Red Spot, a giant storm in the solar system exists on this Planet.
  • Has at least 69 moons, including four large Galilean Moons “Io, Europa, Ganymede, and Callisto” which were discovered by Galileo. “Ganymede” is the largest among them.
  • It has an unclear ring around it.

6. Saturn

  • The second-largest planet in the solar system and a gas giant.
  • Has bright and concentric rings around it which are made up of tiny rocks and pieces of Ice.
  • Saturn can float on water because it has less density than water.
  • Has at least 62 moons and Titan is the largest among them.

7. Uranus

  • Has the third-largest planetary radius and fourth-largest planetary mass in the Solar system.
  • Greenish in colour.
  • Discovered by William Herschel in 1781.
  • Known as “Ice Giant”. The atmosphere of Uranus is composed of Hydrogen and Helium primarily, but it also contains more water, ammonia etc.
  • Has coldest planetary atmosphere in the solar system.
  • Rotates clockwise on its axis like Venus but unlike other planets
  • Has at least 25 moons. Famous moons- Miranda, Ariel and Umbriel

8. Neptune

  • The farthest planet from the Sun.
  • It is also “Ice Giant”. Atmosphere primarily composed of Hydrogen and Helium.
  • Bluish in colour because of Methane.
  • The fourth-largest Planet and the third most- massive planet in the solar system
  • Discovered by Johann Galle and Urbain Le Verrier in 1846. The only planet in the solar system found by Mathematical Predictions.
  • Has known 14 satellites.
  • Famous moon – Triton

Pluto: As per the new definition of Planets determined by the International Astronomical Union (IAU), Pluto had been omitted from the list of planets in 2006. Pluto is considered as a dwarf planet (size between planets and asteroids) now, and it is a member of the Kuiper Belt.

Kuiper Belt: It is a spherical boundary outside the orbit of Neptune containing a number of asteroids, rocks, and comets.

Goldilocks Zone: It refers to the habitable zone around a star where the temperature is just right – not too hot and not too cold – for liquid water to exist on a planet.

Other Space Objects


  • These are small objects; rocks (mostly debris) revolve around the Sun.
  • They are mostly found in the Asteroid Belt which lies between the orbits of Mars and Jupiter.
  • These are also known as Minor planets.
  • Ceres, Vesta, Psyche are some famous and largest asteroids in the solar system.

Meteors and Meteorites

  • These are also known as Shooting stars.
  • Meteors are the small-sized rocky material which is generally formed due to asteroid
    collision and approaching the Earth.
  • Because of Earth’s atmospheric layers, these small rocks burn before reaching the surface.
  • But there are some meteors which do not burn completely and land on Earth’s surface. They are called as Meteorites.
  • Willamette, Mbozi, Cape York, and El Chaco are some meteorites found on the Earth.
  • Lonar lake, Maharashtra in India is supposed to be created by a meteor impact in Pleistocene Epoch.


  • These are shiny, luminous “Tailed Stars”. These are rocky and metallic materials
    surrounded by frozen gases.
  • These are generally found in the Kuiper Belt. They travel towards the Sun.
  • Their tail faces opposite of the Sun and head faces towards the Sun.
  • They become visible when they travel close to the Sun.
  • Halley’s Comet is famous which appeared last time in 1986 and which reappears after every 76 years.
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